Tossing food pellets into the tank, Mr. Herd watches the water suddenly churn with the iridescent colors of swarming trout. Each week, workers at his Cool Water Farms Ltd. in Pickering, Ontario - Canada's largest trout farm - net 25,000 fish, stun and fillet them, and ship them fresh to supermarkets across the country.

With ocean fisheries in decline and global population surging, Herd's brand of high-tech aquaculture - as well as traditional "pond aquaculture" and coastal fish farming - are seen as keys to meeting the world's growing need for food.

"The world is expecting aquaculture production to help bridge the supply-demand gap," writes Meryl Williams in a recent report on global aquaculture for the International Food Policy Research Institution, a Washington think tank.

Hopeful signs include world aquaculture production that more than doubled between 1984 and 1993, reaching 16.3 million tons, reports the United Nations Food and Agriculture Organization (FAO). Growing at 10 to 15 percent annually, aquaculture is expected to rise to between 22 million and 40 million tons by 2010.

Because of the growth in aquaculture, the output of ocean fisheries and aquaculture together will grow 10 to 30 percent by 2010, helping feed a world population 36 percent larger than today's 5 billion-plus people, the FAO predicts.

"Global agriculture production can't keep pace with demand for protein, so people are going to the waterways," says Cyr Couturier, past president of the Aquaculture Association of Canada. "Historically, that's the way people have derived the additional protein they need."

Of course, Herd's trout are a high-end product that costs several dollars a pound by the time they land on Canadian dinner tables. But worldwide, aquaculturalists raise 181 species, including salmon and shrimp, carp and catfish.

Despite a jump in fish-farming in Canada and the United States, Mr. Couturier concedes that both North American nations are relatively small players, far behind when compared with the likes of China, India, and Japan, the world's three largest fish-farming nations.

Feeding China's millions

"Aquaculture in China is developing very fast because of land constraints and food shortages," says Chen Shuping, an aquaculture expert at China's Department of Fisheries. "The government is paying more attention to [inland pond] aquaculture because of environmental concerns and overfishing along the Chinese coast."

China's aquaculture system dates back 2,500 to 3,000 years to the Zhou Dynasty. An Emperor Li prohibited consumption of a common carp, also called "li." So a court official thought up the idea of farming several species of carp in a single pond. Over time, the pond system became integrated with vegetable and livestock production.

Today, farmers cultivate fish in their rice paddies. And China still has large water areas that have yet to be put to use for fish cultivation. Aquaculture production accounts for 55 percent of China's total fish output. But while 6.7 million acres are in aquaculture, just two-thirds of inland water areas and 25 percent of coastal mud flats are utilized.

The emphasis now is on developing the use of ponds and net cages in reservoirs. Mr. Chen says both are intensive methods producing five to 10 times the yield of other methods. They will be needed because fish is a growing part of the diet: Chinese consume 44 pounds of fish per capita per year, up from 20 pounds in 1978. As prosperity spreads in China, more city dwellers are eating more fish, especially luxuries like turtle, shrimp, and eel.

With the decline of ocean fisheries and a growing appetite for seafood, Japan is looking to aquaculture. Japan is already known for having some of the most advanced aquaculture practices in the world. But there is a push to move aquaculture inland as coastal areas become crowded.

Fifty-five percent of fish consumed in Japan is imported. Aquaculture officials want to reduce that percentage. At the same time, traditional small-scale coastal aquaculture is declining because as the work force ages, young people are not following their parents into fish farming .

"To increase [the] self-supply rate, Japan should work more to develop computer-controlled, land-based fish farming," says Yujiro Taniguchi, secretary general of the Japan International Food and Aquaculture Society. Such systems, he points out, would be more efficient and ease pressure on the environment.

European market grows slowly

By contrast, Europeans rank far behind Asians as consumers of fresh fish, but are beginning to pick up the pace. In 1980, the average French family ate 22 pounds of fresh fish a year, falling to 15.4 pounds in 1994, says Philippe Paquotte, an economist at the Paris-based IFREMER, a state-owned research institute that explores and develops ocean resources.

That appetite has grown lately to about 17 pounds. The reasons: The price of fresh fish has dropped 20 percent in the last two years, producers have developed new fresh-fish products that are easy to cook, and French supermarkets have developed new fresh-fish displays to attract customers.

Norway and Greece have taken a strong lead in developing Europe's fish-farming industry. Some 80 percent of fresh salmon comes from Norwegian fish farms, where production has tripled in the last six years. But there are serious limits on further expansion in coastal zones in densely populated Europe - as well as in Japan, China, and the US.

"Finding sites that are favorable to the development of fish farming is a continual problem," says Alain Michel, director of IFREMER's living resource department, which includes aquaculture. "Traditional fishermen, port authorities, environmentalists, and tourists are all positioning themselves to claim the few remaining sites."

Some 60 companies farm 30 sites along the French coast. Most prospective new French sites are on the Mediterranean island of Corsica. But a violent independence movement there has discouraged new investors.

Also limiting the worldwide growth of aquaculture are the problem of pollution and a shortage of appropriate and affordable sites, industry analysts say. Fish farms pack fish so closely that excess food and fish waste can foul surrounding waters. Even with its vast coastline, British Columbia has put a moratorium on coastal salmon-farm development.

"You can't go just anywhere and do aquaculture," says John Malloch, a New Brunswick fisherman turned fish farmer. He started in 1981 with 1,000 fish in a sheltered portion of the Bay of Fundy. The first year, seals broke into the pens. All but 53 fish escaped or were eaten.

Since then, Mr. Malloch has learned a few tricks. He uses a cutout of a man with a gun as a scarecrow to frighten away seals. Today his operation ships 500 tons (about 150,000 Atlantic salmon) annually, making him one of the top independent producers in Canada. Malloch says aquaculturalists are increasingly burdened by overly strict environmental regulations that make it difficult to start up a fish farm.

Future fish-farming operations will be offshore, outside critical environmental zones, experts say. They will involve fish cages able to submerge during heavy storms and then pop back up. That will require new technology but will be essential to meeting the world's food needs.

"As population grows, and as you saturate inshore areas with development, you're going to have to move offshore to farm fish," says James McVey, director of the National Sea Grant College Program, a division of the US Department of Commerce that funds underwater studies.

Malloch has doubts. "If you can figure out how to put pens in the open ocean, you might be able to expand aquaculture quite a bit," he says. But "that seems like a long way off to me."

Farming the open ocean

Dave Brittain, chief of London-based Dunlop Aquaculture, already sells "tempest" cages - flexible cages that can be grouped together in open water.

Already, Dunlop's cages - nets supported by huge rubber tubes of the sort used to pump oil from supertankers - are in operation in the Mediterranean Sea and off the coast of Ireland in areas exposed to open ocean. The cages, he says, can withstand waves higher than 45 feet.

At the same time, he says, nations will have to agree to stop overfishing coastal waters for the two dozen or so species that currently are the focus of the world's fishing fleets.

"We will have to have a blend of ocean fisheries and new methods and means of aquaculture production," Mr. McVey says. "That's the only way we're going to do it."

* Monitor writers Sheila Tefft in Beijing, Gail Chaddock in Paris, and Miharu Hasegawa in Tokyo contributed to this report.